Tectonic setting of the Youjiang giant tin belt, South China: New findings from the Pingna W-Sn deposit

Growing evidence suggests that extensional/transtensional settings are favorable for the formation of tin deposits, yet the underlying geodynamic mechanism remains equivocal. The Pingna W-Sn deposit, found in the underexplored interior of the giant tin belt within the Youjiang Basin, South China, offers a unique opportunity to explore and better constrain the current geodynamic model for tin mineralization. This deposit, composed of NW- to NWN-striking vein swarms with W-Sn mineralization, is hosted in the Middle Triassic clastic rocks without igneous rocks near its mineralization. Structural analysis indicates that the Youjiang fold-and-fault belt and the ore-related structures in the Pingna deposit experienced five deformation phases (D₁-D₅). The pre-ore NE-striking compression (D₁; σ_v=σ₃) initiated fault-fracture meshes, followed by NE-striking extension (D₂), while NW-striking compression (D₃; σ_v=σ₂) enhanced the vertical connectivity of the meshes. Syn-mineralization E-W extension (D₄; σ_v=σ₁) facilitated upward through-going flow and hydrothermal fluids infilled the meshes, forming a fault-vein system. The mineralized veins were cut across by post-ore WNW-striking oblique fault with sinistral and normal components (D₅). The meshes dictated Sn-W orebodies localization. Hydrothermal veins formed in three stages:(I) muscovitization-bordered tin-dominated quartz vein swarms along the Pingna fault; (II) W-dominated lit-par-lit vein system; and (III) barren calcite veins crosscutting the former veins. The Pingna W-Sn mineralization formed during the Late Cretaceous as constrained by the cassiterite (Cst1) U-Pb age of 95.6 ± 2.4 Ma (2σ, MSWD=1.2), muscovite (Ms1) ⁴⁰Ar-³⁹Ar plateau age of 93.9 ± 0.1 Ma (2σ, MSWD=1.7), and molybdenite Re-Os age of 92.9 ± 1.2 Ma (2σ, MSWD=0.3). Outward lateral zoning of the Sn-W mineralization, as well as associated muscovitization and silicification implies the epicenter of hydrothermal fluid is near the No. II vein swarm. Contemporaneous felsic dykes coupling with the inferred intrusions demonstrate that the Pingna deposit is a distal hydrothermal W-Sn deposit. The releasing bend of the NW-striking Pingna fault controlled the distribution of tin-dominated mineralization, while the anticlines controlled the tungsten-dominated mineralization. Our findings suggest that the localization and formation of the Pingna W-Sn veins were controlled by Late-Cretaceous regional transtensional stress field and polyphase deformation, rather than previously proposed local extension of the Youjiang Basin. The discovery of the Pingna W-Sn deposit highlights the interior of the Youjiang Basin as a promising area for tungsten-tin exploration.